1. Field of the Invention
The present invention relates generally to a communication method and apparatus between a User Equipment (UE) and an evolved NodeB (eNB) in a wireless communication system, and more particularly, to a method of handling multi-eNB connectivity when a radio link failure of the UE is generated.
2. Description of the Prior Art
A 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) (Release 11 and earlier) system is based on a single connectivity model in which one UE is connected to one eNB. In such a system, handling Radio Link Failure (RLF) is based on the detection of the RLF by the UE. A link with the eNB is in an RLF state, so that the RLF is based on additional actions performed by the UE without any notification of the RLF to the eNB.
FIG. 1 is a diagram illustrating handling of the RLF in a mobile communication system, and FIG. 2 is a flowchart illustrating a process of handling the RLF in a mobile communication system.
Referring to FIG. 1, in order to detect the RLF, the UE may perform a process known as Radio Link Monitoring (RLM). The UE may measure a Block Error Rate (BLER) of a Physical Downlink Control Channel (PDCCH) during a predetermined time period. When the BLER drops below a preset threshold (Qout) during the predetermined time period, an out-of-sync indication may be generated in a physical (PHY) layer. When a preset N310 number of successive out-of-sync indications are reported to a Radio Resource Control (RRC) layer by the PHY layer, an RLF timer T310 is started by the RRC layer. At this time, when the RLF timer expires, the RLF may be declared. Further, when the timer T310 starts, and then a predefined N311 number of successive in-sync indications are reported by the PHY layer, the timer T310 may stop.
Referring to FIG. 2, a UE 210 generates the RLF in a first link, in step 251. Further, after the timer T310 expires, the UE 210 may declare the RLF. In the declaration of the RLF, the UE 210 may stop all uplink transmission to avoid a possibility of the generation of uplink interference, and deactivate all radio bearers. Further, the UE 210 scans and is synchronized with a target cell 240, in step 252, and transmits a connection re-establishment request message to the target cell 240, in step 253. When the target cell 240 already has UE context, in step 254, the target cell 240 transmits a connection re-establishment success message to the UE 210, in step 255. After the successful re-establishment, the connection between the UE 210 and the target cell 240 resumes, in step 256.
However, when the target cell 240 does not have the UE context, in step 257, the target cell 240 transmits a connection re-establishment failure message to the UE 210, in step 258. After the connection failure, the UE 210 enters an idle mode, in step 259, and the UE 210 eventually triggers a new connection with the target cell 240, in steps 260 and 261. This may cause an application level connection to terminate, and thus, is very undesirable. Further, it should be noted that the target cell 240 has the UE context only when a handover is performed before the RLF is generated. Accordingly, a re-establishment process may be successfully achieved only in such a scenario.
When the preset maximum number of random accesses has failed, the existence of the system RLF may be also triggered. Further, when the preset maximum number of RLF retransmissions is performed, the RLF may be also triggered.
The related art discloses content in which the UE scans a proper target cell after declaring the RLF, and transmits an RLF indication indicating that the RLF is generated in a previous eNB (by notifying of an ID of the previous eNB) to the selected target cell in a single connectivity system (3GPP LTE Release 11 and earlier). The target cell transmits the information to the previous eNB. The previous eNB having received the information may improve handover parameters, which may cause the RLF and other radio link parameters.
The terms eNB and cell may be exchangeably used throughout.